Optical inspection allows for rapid and effective identification of defects in semiconductor objects, which include (but are not limited to) semiconductor wafers, reticles, mask patterns, and other items that are the result of or are used in fabrication of miniaturized electronic devices.
Various illumination and imaging systems have been proposed for optical inspection tools. For example, some systems use lamps or lasers to illuminate the object under inspection, with a detector or an array of detectors used to image areas of interest on the object.
FIG. 1 illustrates a common scheme of dark field illumination typically used in semiconductor inspection systems comprising two-dimensional detectors. An illumination source 104 illuminates a semiconductor object 12 at an oblique angle, often using Kohler illumination. The impingement angle Φ is generally very acute relative to the object plane.
The source emits light 108 with a predefined angular distribution at the focal plane of a lens 106 or other optical elements in the illumination path. The light passes through the lens 106 (and/or other components) and impinges the semiconductor object 12, which is located at the other focal plane of the lens.
As shown in FIG. 2, since the illumination source angular distribution is usually circular and due to the oblique illumination angle, the illuminated area 110 on the semiconductor object is an ellipse. At a typical illumination angle of 15°, the aspect ratio of the ellipse is about 1:4. However, the imaged area 112 of an inspection tool is usually in the shape of a square or a slightly rectangular square (i.e. a rectangle with aspect ratio not far from one). All the illuminated area outside of the imaged area does not reach the tool's detector(s) and therefore can be considered as an illumination loss.
Another problem recognized by the current inventors regarding oblique Kohler illumination in inspection tools is illustrated at FIGS. 3 and 4. FIG. 3 shows a ray trace for two points (A and B) of a non-oblique illumination source 104, i.e., a configuration where object 12 is perpendicular to incoming illumination. In FIGS. 3 and 4, the dashed lines represent ray traces from point A, while the dotted lines represent ray traces from point B. Each point illuminates the same area, A′B′, on the object, at different impingement angle (ΦA1 for light from point A, ΦB1 for light from point B).
However, when the illumination is oblique relative to the object, as shown at FIG. 4, the area of each illumination point is different. The illumination from point B at the source impinges the object at a more acute angle (ΦB2) than the angle (ΦA2) for illumination from point A. A closer view of the impingement angles is shown in the magnified view of FIG. 4A. Therefore area B′ illuminated by light from point B is larger than area A′, representing the area illuminated by light from point A.
A diagram of the illuminated area 110 is shown in FIG. 5; as before, in this example, a circular angular distribution of illumination for an object at an oblique angle results in an ellipse-shaped illuminated area. In this figure, the areas with less dense cross-hatching receive illumination at higher intensity, with the highest intensity in the center. Light from all parts of the source illuminates the center of the illuminated area on the object. However, the periphery of the illuminated area does not receive light from all parts of the source since light from different parts of the source impinges at different angles (with different resulting areas). Therefore, the illuminated intensity is lower moving outward from the center, especially along the major axis of the ellipse.
FIG. 5 also shows two exemplary imaged areas 114A and 114B. If larger area 114A is imaged, the resulting image will likely be degraded due to non-uniform illumination. If the imaged area is set to be smaller than the center of the illuminated area (rectangle 114B), illumination may be more uniform at the cost of higher illumination loss.